1. Field of the Invention
The present invention relates to a heat-treating apparatus for treating a wafer-like material such as a semiconductor wafer and a liquid crystal wafer, particularly, to a vertical heat-treating apparatus comprising a plurality of reaction tubes arranged in a vertical direction for heat-treating wafers.
2. Description of the Related Art
In recent years, vigorous researches have been made in an attempt to develop a vertical heat-treating apparatus used in the thermal diffusion step or film-forming step in the manufacture of a semiconductor device because a vertical apparatus permits saving the required space and energy, facilitates the treatment of a semiconductor wafer having a large diameter, and is capable of readily coping with the automation of the apparatus.
The vertical heat-treating apparatus comprises various heat-treating devices such as an oxidation device for forming an oxide film, a metal film, a monocrystalline film, or for performing an impurity diffusion, a CVD device, an epitaxial device and a diffusion device. In each of these heat-treating devices, a coil heater is wound about a reaction tube standing upright so as to heat semiconductor wafers housed in the reaction tube to 500.degree. to 1250.degree. C., thereby to form a film on the wafer or to diffuse an impurity into the wafer. For performing these treatments within the heat-treating device, it is necessary to use a wafer supporting body which has a high heat resistance and does not generate an impurity under high temperatures. In general, a quartz boat is used as such a wafer supporting body meeting the requirements given above.
In performing the heat treatment, a carrier supporting, for example, 25 semiconductor wafers is taken out of a carrier stocker and moved onto a wafer transfer device. Then, the semiconductor wafers are transferred onto the quartz boat which is held horizontal. The quartz boat is then converted from the horizontal state into the vertical state by a boat driving mechanism. Under this condition, the quartz boat supporting the semiconductor wafers is moved onto a boat transfer device and, then, into one of a plurality of heat-treating furnaces.
The wafer transfer device is constructed such that the carrier vertically supporting a plurality of parallel-arranged semiconductor wafers is taken out of the clean wafer stocker. Then, the supporting body is elevated from below the carrier so as to collectively support the semiconductor wafers in an upper position. The collectively supported wafers are held by a holding means and moved horizontally onto a quartz boat disposed horizontally near the holding means. Then, the holding means is moved downward so as to transfer the wafers onto the quartz boat, followed by releasing the holding means. The operations described above are reversed in the case where the treated semiconductor wafers are brought back onto the carrier. What should be noted is that the driving mechanism is frequently operated in the steps of transferring the semiconductor wafers. Naturally, dust or fine particles are generated by the operation of the driving mechanism, making it very difficult to prevent the dust or fine particles from being attached to the semiconductor wafers, even if the carrier supporting the wafers is stocked in a clean carrier stocker.
The dust or fine particles is also generated during the quartz boat transfer from the boat driving mechanism into the heat-treating furnace, leading to the dust attachment to the wafer, etc. Naturally, contamination of the wafer leads to a low yield of the semiconductor device. The difficulty is rendered more and more serious in accordance with the recent requirement for a higher integration degree of a semiconductor device.